Transistor-transistor logic (TTL) is a logic circuit family currently in widespread use, is relatively inexpensive, and TTL is also the fastest logic family which operates the individual transistor elements in the saturation region. State-of-the-art integrated circuit design makes wide use of the TTL family due to its many advantages and operational capabilities.
One current disadvantage in using the TTL family, however, is its inability to operate at very high speeds, that is TTL circuits are generally unsable to operate at clock rates of over 100 MHZ. There are, however, many circuit applications where an operational rate greatly exceeding 100 MHZ is necessary, which circuit applications would preferably be designed with TTL circuits, due to its relatively low cost and wide availability. Because operational speeds in excess of 100 MHZ, are not possible with TTL circuits, other alternatives to attain high speed operations have been considered.
One common alternative to the use of TTL circuits in high speed applications is the use of Emitter-Coupled Logic (ECL). The ECL family is the fastest form of integrated circuit logic currently availale, and is sometimes referred to as current-mode logic, current-steering logic, or non-saturating logic. High speed operation of ECL circuits is possible because transistor saturation is prevented by the basic circuit design, and the fact that the transistors are not in saturation permits faster switching operations. Many high speed applications make use of ECL circuitry, one of which is described in U.S. Pat. No. 4,238,774 by Frederich D. Lehman, and assigned to the same assignee CPT Corporation, as is the instant application. In U.S. Pat. No. 4,238,774, there is described a drive circuit for a high resolution cathode ray tube display. The improved drive circuit described in this patent permits the operating frequency of the horizontal drive for the CRT to be increased by a factor of four, and the operating frequency of the video drive for the CRT to be increased by a factor of ten. Through use of the ECL circuit design, the drive circuit described in the '774 patent is able to achieve horizontal operational frequencies of approximately 50 KHZ and video drive operational frequencies of approximately 200 MHZ.
Although use of ECL, as described in U.S. Pat. No. 4,238,774 permits extremely high speed operation to be achieved, several problems arise when using ECL circuit designs. First, ECL is substantially more expensive than comparable TTL circuits. For example, a high speed clock designed using ECL can be three-five times more expensive than a comparable design in TTL. Second, certain circuit designs which are generally an "off-the-shelf" purchase in TTL are often a custom order in ECL due to the somewhat lesser availability of standard ECL designs, as opposed to the widespread availability of TTL designs. As is well known in the electronics industry, the necessity to "custom order" any device often results in substantial delay, as well as the attendant increased cost. The third disadvantage, and perhaps the most important, is the fact that ECL circuit designs generally consume substantially more power than do comparable TTL designs, which power consumption is completely unsuitable for certain applications.
It is, therefore, a general object of the instant invention to provide method and apparatus for effectively increasing the operational speed of digital circuitry, while maintaining, to the extent possible, the use of TTL devices.
It is another object of the instant invention to achieve high speed operation of digital circuitry, while avoiding, to the extent possible, use of ECL circuit elements.
It is a further object of the instant invention to provide a high speed digital circuit design, which is low in cost, low in power consumption, but capable of attaining very high operating frequencies.